Understanding The Expansion of Universe
At a fundamental level, the expansion of universe is a fundamental property of spatial measurement on the largest measurable scales of our universe. The distances between cosmologically relevant points increases as time passes leading to observable effects outlined below. This feature of the universe can be characterized by a single parameter that is called the scale factor which is a function of time and a single value for all of space at any instant (if the scale factor were a function of space, this would violate the cosmological principle). By convention, the scale factor is set to be unity at the present time and, because the universe is expanding, is smaller in the past and larger in the future. Extrapolating back in time with certain cosmological models will yield a moment when the scale factor was zero, our current understanding of cosmology sets this time at 13.75 ± 0.11 billion years ago. If the universe continues to expand forever, the scale factor will approach infinity in the future. In principle, there is no reason that the expansion of the universe must be monotonic and there are models that exist where at some time in the future the scale factor decreases with an attendant contraction of space rather than an expansion.
Read more about this topic: Metric Expansion Of Space
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